Question

What is the amount of lime $Ca{\left(OH\right)}_2$ required to remove the hardness in $60$ litres of pond water containing $1.62$ mg of calcium bicarbonate per $100$ mL of water?

• A. $0.444$ g
• B. $0.434$ g
• C. $0.464$ g
• D. $0.454$ g

Answer 1

The correct option is A $0.444$ g

$Ca(HC{O}_3{)}_2+Ca(OH{)}_2\to 2CaC{O}_3+2H_{2}O$
Pond water contain 1.62 mg of calcium bicarbonate per 100 mL of water.
60 litres of pond water will contain $\frac{1.62}{1000}\times \frac{1000}{100}\times 60=0.972$ g of calcium bicarbonate.
The number of moles of calcium bicarbonate are $\frac{0.972}{162}=0.006mol$.
They will require 0.006 moles of calcium hydroxide.
The mass of calcium hydroxide required is $74\times 0.006=0.444g$.